HEAT-SHRINKABLE ARTICLE

Abstract: A composite material having heat-shrinkable fibres comprising LLDPE, characterised in that the LLDPE has a weight average molecular weight of from 55,000 to 130,000 and a crystallinity less than 60 %.

Description: Heat-Shrinkable Article The present invention relates to an article having im proved heat-shrink characteristics, in particular to one combining improved heat-shrink characteristics and excellent mechanical properties. Heat-shrink articles find particular use in environmental protections of s u b strates such as cab les and pipe s, particularly splices in telecom munications cables. For exam ple, a cable splice case is expected to have a life-time comparable to that of the cables it joins, typically 25 years, under unfavourable environmental conditions. Products for use as cable splice cases m ust pass rigorous te m perature and pressure cycling tests if their purchaser is to have confidence that they will perfor m satisfactorily over these long life times. Designing a s plice case is therefore a difficult m atter, and careful choice of materials must be made. He at- shrinkable materials have been widely use d for environmental protection because they can be produced over-sized, easily installed, and then shrunk into tight, sealing engage ment with the sub strate to be protected. They are frequently internally coated with a heat-activatable adhesive such that the heating required for installation causes both shrinkage of the article and activation of the adhesive. In this way, an excellent environmental seal can be produced. Recently, heat-shrinkable composite materials have begun to re place monolithic heat-shrinkable sleeves in particular in the teleco m m unications cable accessories m arket. S uch co m posite materials have the advantage of su perior mechanical properties, in particular split-resistance. They are manufactured by incorporating a heat-shrinkable fabric within a matrix material; shrinkable fibres of the fabric drive shrinkage of the product and provide split- resistance perpendicular to the shrink direction, glass fibres of the fabric provide additional strength, and the matrix material renders the prod uct impervious. Whilst such composite products are ideal for manv applications there are occasions when the temperature and quantity of heat required for installation are too great Problems may therefore arise where an open-flame torch, which can supply vast quantities of heat, is considered undesirable, and/or where the cable or other substrate around which the heat shπnkabie article is to be shrunk is easily damaged by heat For example, optical fibre cables are generally rather small and do not carry a large mass of highly heat-conducting copper They are therefore easily damaged by heat It is desirable therefore that a technique be found for joining or sealing them that avoids high temperature and prolonged heating It is generallv recognised that of the few broad types of materials used for the formation of heat-shrinkable articles, LLDPE thin film has advantages where low shrink temperatures are required Unfortunately, that information is not very helpful since a vast number of materials of widely differing properties are known as LLDPE, and what applies to thin film for temporary packaging cannot be assumed to apply to the generally thick-walled materials required for long-term environmental protection of, for example, outside telecommunications cable plant Nonetheless, a brief review of LLDPE may be made The table below shows just some of the LLDPEs available Table 1 We have found that the great majority of these LLDPEs does not overcome the problems that we wish to solve and that the patent literature does not give much guidance EP-A-0225152 (Raychem) discloses a heat-shrinkable composite article for environmental protection of telecommunications splice cases The composite contains a heat-shrmkabie fabric, the shπnkable fibres of which may be LLDPE Unfortunately, no guidance is given concerning the LLDPE and the material is merely mentioned as one grade of poiethylene US 4554202 (Kamei et al) discloses a packing cloth comprising a woven fabric made from a thread obtained by melt extrusion of LLDPE having a density of less than 0945g/cm2 and having a branched short chain This material is stretched at a temperature of less than 120 ° C The cloth apparently has an improved tear strength and notch propagation resistance The LLDPE is said to have a branched short chain having ten or fewer carbon atoms The material has a melt flow index of from 005 to 3g/10 minutes The LLDPE maybe extruded through a slit of a dye in the form of a film and, after cooling, stretched at a temperature preferably from 70 to 110°C at a stretch ratio of between 2 and 9 The resulting fabric may be laminated onto a surface of a plastic film layer having heat shπnkability identical or similar to that of the fabric itself DE 4039965 (Stewing) discloses a heat-shrinkable sleeve for sealing a cable splice, and having high tear-resistance. For high shrinkage rates the fibre used is a polyethylene, preferably a high density polyethylene or LLDPE, with an average molecular weight and a close molecular weight distribution of 3 to 5- LLDPE is disclosed in GB-A-2220667 (Bowthorpe-Hellermann Limited). This specification discloses a reinforced polyethylene composition comprising low density polyethylene and, as a reinforcement, finely divided fibres of high density polyethylene. The LLDPE used may be that known by the trademark Stamylex 08- 026, which has a density of 0.911g/cm2 and a melt flow index of 2.2g/10 minutes. A non woven fabric formed by heat-bonding filaments of LLDPE is disclosed in US 4981749 (Kubo et al). The LLDPE has about 1 to 10 weight percent of octene-1, has a density of 0.900 to 0.940g/cm 2 , and a melt index of 5 to 45g/10 minutes. A similar material is disclosed in US 5068141. LLDPE in sheet form is disclosed in GB-B-2019412 (Raychem). There a material is used for splice case protection comprising a cross- linked polymeric composition of LLDPE homo or copolymer which, prior to crosslinking, is characterised by a density at 25°C of 0.940g/cm 2 or less and a linearity expressed in terms of pendant methyl groups per thousand carbon atoms of the polymer chain of less than 30 with not more than 5 pendant chains greater in length than C8 per thousand carbon atoms of the polymer chain. Preferred polymers have a molecular weight distribution index of below 8, preferably from 3 to 8, e.g 3 to 7. One LLDPE given as an example is that known by the trademark Sclair 8107 UV1, which material has a degree of crystallinity of 44%, has seven pendant methyl groups per thousand carbon atoms and less than 7 short chain branches per thousand carbon atoms. LLDPE is mentioned in general terms in GB-A-2181437 (Kabeimetal) where an electrical cable or a tube is formed from cross-linked LLDPE having a density of from 088 to 095g/cm^ Heat-shrinkable tubes and sleeves formed from crossimked LLDPE are disclosed in DE-A-3243102 (Kabelmetal) No explanation as to the choice of material is given except for the statement that very short branches in the chain structure of the LLDPE allows the chains to slide without any elastic recovery forces being built up Various references exist to the use of LLDPE for the production of thin packaging films US5110686 (Showa Denko KK.) discloses a low temperature heat shπnkable film comprising LLDPE having a short branched chain and a density of 0935g/cm.2 LLDPE is apparently chosen because a draw ratio of the film of from 3 to 6 is possible Draw ratios above 6 apparently lead to breakage A fibre draw ratio of 6, however, would lead to a much smaller recovery ratio of a finished composite and a product having such smaller recovery ratio would m general be unacceptable for the purposes of the present invention An LLDPE known by the trademark Dowlex 2045 is disclosed in US 4514465 (WR Grace and Company) There a five layered thermoplastic film for use as storm windows is disclosed, one layer of which comprises LLDPE A definition of LLDPE is given namely copolymers of ethylene with one or more comonomers selected from C4 to CIO alpha olefins such as butene-1, octene etc m which the molecules of the copolymers comprise long chains with few side chains, branches or crossimked structures LLDPE as defined in that specification has a density usually from 09 to 0925g/cm2 and has a melt flow index from 01 to about 10g/ 10 minutes US-4923722 (Okura Industrial) discloses a heat-shrinkable composite film having a core layer formed of a straight chain low density polyethylene having a density of from 089 to 0905g/cm2 The material used can apparently be shrunk over a wide temperature range to give a tight package free from wrinkles and having high tear strength A crossimked LLDPE is disclosed in US5023143 (WR Grace) Materials from Dow Chemical Company, Union Carbide and Exxon are mentioned but no further information is given than a preferred density between 0.910 and 0.935g/cm2, and a melt flow index from 1 to 6. The nature of LLDPE is specified in slightly more detail in US4820557 (WR Grace). Here LLDPE refers to copolymers of a major amount of ethylene with a minor amount of one or more comonomers selected from C3 to C10 or higher alpha olefins, such as butene-1, pentene-1, hexene-1, octene-1, etc in which the molecules thereof comprise long chains with few side chains or branched structures achieved by low pressure polymerisation. The side branching which is present will be short as compared to non-linear polyethyienes the molecular chains of a linear polymer maybe intertwined, but the forces tending to hold the molecules together are physical rather than chemical and thus maybe weakened by energy applied in the form of heat. Specific materials mentioned include Dowlex 2045.03 and DEFD 1630 (trademarks). Further references to LLDPE in the context of shrinkabie film include US4617241 (WR Grace), US4514465 (WR Grace), US4590124 (WR Grace), GB-A-2135240 (WR Grace) and US5079051 (WR Grace). Many of these references relating to shrinkabie thin film are concerned with the formation of multi-layer structures, it being through the combination of various films that the desired properties are allegedly achieved. No consideration appears to be given to the precise nature of the LLDPE even though a wide variety of grades is available. In spite of this, we have found that in order to produce a satisfactory product that will meet the demanding requirements of, for example, outside plant cable sealing great care must be taken in materials selection: materials must be chosen bearing in mind manufacture, heat installation, and long-term performance. We have found that, in general, fibre-reinforcement is desirable and that a fibre should be stretched 8, 9, 10 fold during manufacture if it is to provide the desirable degree of shrinkage in the final product. A particular draw force is desirable at the limit of this stretching and also after subsequent recovery of say 5X (which ratio is expressed as the pre-shrmkage length divided by the post-shrink length) This resid ual recovery force will ensure a safety m argin after the desired dimensional change has occurred If the draw force is insufficient then the fibres will not sufficiently drive shrinkage of the overall product, or will inhibit shrinkage of other active components, and if the draw force is too high then fibre breakage is likely A further re q uire m ent is that the fib re s shrink at a s ufficiently low temperature and require a sufficiently small amount of heat Most of the LLDPEs available do not meet these requirements We have found that LLDPEs having a certain molecular weight and a certain crystallinity and preferably a certain T m value allow a prod uct to b e m ade having dram atically su perior perfor m ance, especially during installation Fibres may be made from one or more of these preferred LLDPEs alone, or that material m ay be blended with other polymers Exam ples of other polymers include other polyolefmes such as HDPE and other LLDPEs, and materials such as EPDM The extent of the improve ment will depend , of course, on the proportion of the total material that consists of the selected LLDPE, b ut benefits are noticeable over a wide range of com positions By choosing an LLDPE and by selecting any of the characteristics mentioned herein, particularly M and crystallinity, we believe we have avoided to some extent the dichotomy between low tem perature perfor mance and high strength or high recovery ratios It is in principle simple to prod uce a material that shrinks at a low tem perature, provided one is prepared to accept a low degree of shrinkage and low shrink force Whilst we do not wish to be bound by any theory, we believe we have achieved a useful compromise between the energy required to release the shrinkage forces and the size of those forces as follows We have selected a reasonably high molecular weight and as a result benefit from long molecular chains whose entangle ment provides the desired strength We are able to reduce the amount of heat required for shrinkage by reducing the crystallinity of the material such that a smaller latent heat of crystallisation is required before the oriented molecules can revert to their ther modynamically stable conformations. Surprisingly, we have been able to achieve this by using a material that is predominantly designed for use as a cast film and which was not intended as a fibre-forming material. Thus, the present invention consists in a composite material having heat-shrinkable fibres comprising LLDPE, characterised in that the LLDPE has a weight average molecular weight (Mψ) of from 55,000 to 130,000 and a crystallinity less than 60%. Molecular weight parameters Mw, Mn etc and molecular weight distribution (D) etc, were carried out using gel permeation chromatography (GPC) utilising the following conditions. Solvent : o-dichlorobenzene at 140°C at a flow rate of 1 ml/minute Sample : 200mlitres of 0.3% concentrations of polymer in o- dichlorobenzene. Sample filtered before injection. Internal flow marker incorporated Detector : Infra-red detector measuring at 3.42mm. Columns : 2x60cm. 10mm Polymer Laboratories Mixed Gel Data Reduction : Polymer Laboratories Caliber software Molecular Weight Standards : Polystyrene narrow molecular weight standards. A 3rd order polynomial was fitted through the calibration points. Sample results were expressed as LDPE equivalents using the Mark-Houwink equation with values for the K and a constants as follows. Polystyrene LDPE K 13.8 x 10-5 50.6 x 10-5 a 0.7 0.7 The LLDPE preferably has a M w less than 120,000, more preferably less than 110, 000, particularly less than 100,000 and preferably greater than 60,000 more preferably greater than 65,000 in particular greater than 70,000. For most uses we prefer that Mw is from 70,000 to 90,000. The crystallinity of the LLDPE is preferably less than 55, more preferably less than 50, particularly less than 45 especially less than 40%. Generally a crystallinity of at least 15, preferably at least 20, especially at least 30% will be desirable if the material is to have a satisfactory shelf-life. This will of course depend on where in the world the material is to be used, and on the way on which it is stored. Various other characteristics of the LLDPE may be mentioned as preferred. It preferably has a Tm value of from 60 to 130°C, more preferably 80 to 128°C, especially 100 to 128, more especially 125 to 128°C. In general we prefer that the material crystallises with imperfect crystals in order that some melting occurs in the early stages of the heating carried out for heat recovery. In this way the so-called induction time (the heating time during which no shrinkage is visible) can be reduced. As a result the total heating time might be reduced, but in any case a more controlled and steady shrinkage can be carried out in practice. The extent to which the crystallites of the LLDPE are perfect can be readily observed from a differential scanning calorimetry plot where imperfect crystals manifest themselves as a peak having a broad tail towards the low- temperature end. This can be controlled to some extent by the molecular weight of the LLDPE and by the number and type of comonomer involved in its manufacture. We prefer that the LLDPE have from 2 to 30, more preferably from 5 to 25, particularly from 10 to 20 branches per thousand carbon atoms, which branches are preferably derived from a C6 to Cio comonomer, particularly octene. The LLDPE preferably has a molecular weight distribution of 2- 6, more preferably from 3 to 5, particularly from 3.5 to 4.2. The LLDPE can also be characterised at least in part by its melt flow index (MFI) and this is a function at least of molecular weight. A preferred range is from 0.7 to l Og/ 10 minutes, preferably at least 0.9 , more preferably at least 0.2 , most preferably at least 3 , and prefer ab ly le ss than 8 , more preferably less than 6. MFI measurement conditions herein are ASTM D 1238 , 190 °C/2.16Kg. In order to render it shrinkabie, the LLDPE is stretched by a s uitable amount which will depend on the amount of shrinkage desired in the final product, on the other components of the product with which it is u sed, and on the form in which it is used. For m any purposes, we prefer a stretch ratio of at least 6X, preferably at least 8X, particularly at least 10X. In general we prefer that the fibres be crosslinked particularly by radiation. This may be carried out before or after stretching. A suitab le irradiation dose will depend on the precise nature of the polymer and on the extent to which it has been stretched, although 1 0 to 25 , preferably 15 to 25, more preferably 1 8 to 22 Mrads may be taken as a guide. Where the LLDPE (or blend containing LLDPE) is crosslinked some of the characteristics mentioned above might not be determinable directly. In general, the quantities given herein by means of which the LLDPE is characterised refer to the polymer before crosslinking and it will generally be possible to estimate the pre-crosslinking values from analysis of a crosslinked material. The table below lists some preferred materials for use as the LLDPE of the present invention. Table 2 C= CO CO I CO ZΌ CD Of the materials in the above table at present we prefer those known by the following trademarks: Stamylex 1 046F, Stamylex 1 066F, Dowlex 2288E. It may be noted that none of the materials in the above table are marketed as fibre-grade materials. We prefer that the composition of the invention is shrinkabie by virtue of the heat shrinkabie fibres co m prising LLDPE. It is possible however that these fibres be present as part of a composite where recovery is at least principally driven by other components such other components may comprise other fibres or may comprise sheet material. For most purposes the fibres will for m part of a fabric since this aids formation of the prod uct and allows the shrinkabie LLDPE to be combined with other fibres contrib uting other functions to the product. For example, the LLDPE fibres may be combined with glass or other high-strength, non-shrinkable fibres which contrib ute to the overall strength of the product. This may be achieved by providing a weave comprising a warp or weft comprising the shrinkabie LLDPE fibres, and respectively a weft or warp of glass. Either or both types of fibre may be present as monofilaments or as m ulti filaments. Where m uitifilament fib res are used, interstices thereof may be blocked, for example by the provision of hybrid fibres. See EP-A- 0324630. In general, when the com posite material is to be used for environmental protection of a cable etc the composition will be provided as a tubular or wraparound sleeve where the recoverable fibres run substantially circu mferentially. In this way, those fibres can drive recovery to cause engage ment of the material with the cable to be protected and can also provide protection against longitudinal splitting. Instead of a woven fabric a knit, braid or non- woven fabric may be provided comprising the LLDPE fibres. In most cases the fabric will be made impervious by combining it with a matrix material, which may comprise one or more layers of polymeric material applied to the fab ric. The matrix material preferably per meates interstices of the fabric to produce a truely unitary structure. The shrinking fabric preferably defor ms heat- softened m atrix material such that the unitary structure is retained with no delamination. The degree of imperviousness will depend on the final use of the product, b ut we prefer that it be im pervious at least to liq uids and preferably also to air. Most polymeric materials are slightly pervious to moisture vapour b ut the rate of permeation of moisture through a com posite of the sort describ e d will b e sufficiently low that a product life time of many years will readily be achieved. Where extre mely low levels of moisture penetration are necessary the com posite m aterial m ay include a moisture-vapour b arrier layer such as an alu miniu m or other metal foil. Such a b arrier layer may be applied after the application of a polymeric layer and on the side of it facing away fro m the fabric, and preferably on the side closer to the sub strate around which the material is to be shrunk. Various further layers may be provided for example a hot-melt or otherwise heat-activatable adhesive. Such an adhesive will, in general, be inwardly-facing in the final product so that it causes a bond to be formed between the composite material proper and the cable etc to be protected. V arious m aterials may be used for the so-called m atrix material. At present we prefer low density polyethylene which may b e cross-linked. A low degree of crosslinking will in general be sufficient since the principal purpose of crosslinking is merely to stop excessive flow during heat recovery. As with the fibres, crosslinking may be carried out by irradiation, b ut here a dosage of from 0.5 to 5, preferably 1 -4, more preferably 2.5 to 3.5 Mrads will in general be sufficient. The low density polyethylene or other layer may be, for example, from 0.1 to 1 0, more preferably 0.1 to 1.0, especially 0.2 to 0.4m m thick. We have found that quite thin layers can be sufficient. In this way we are ab le to reduce further the amount of heat required for installation since the thermal mass of the product is reduced. Where the fabric is coated on each side with a polymeric layer we prefer that the layers be of about the same thickness, although they can differ by for example a factor of from 1 .5 to 3. For example, each layer may be about 0.25 m m thick. It is preferred that the outer layer be unstressed since an unstressed layer is less likely to split when heated. The thickness of the fib res and their weave (or other fabrication) density can be selected according to the recovery force, recovery ratio, and general ruggedness desired of the product. For many purposes a recoverable fibre diameter of 0.2 to 1 .0 , more preferably 0.2 to 0.6, especially 0.3 to 0.5 m m will be suitable. A fairly open design of fabric is preferred and for many uses a satin weave of float from 2 to 7 especially from 3 to 6 is expected to be satisfactory. The composite material of the invention will generally be su pplied in the for m of a sleeve, particularly in the form of a wraparound sleeve having some for m of closure means running along opposing longitudinal edges thereof. Such closure means are com monly known as "rails", which can conveniently be held together after the product has been wrapped around a cable by means of a channel that is C-shaped in cross section. This allows the material of the invention to be installed around a cable without access to a free end of the cable. Whilst the invention has many uses, we foresee its particular use in a method of environmentally protecting a substrate such as an optical fibre cable or a telecom munications cable splice, in which method the composite material is positioned around the splice such that it extends from one intact cable jacket at one side of the splice to intact cable j acket at the other side of the splice. Once it is in position it may be heated using for example a hot air gun.

Claims: CLAIMS 1. A composite material having heat shrinkabie fibres comprising LLDPE, characterised in that the LLDPE has a weight average molecular weight of from 55,000 to 130,000 and a crystallinity less than 60%. 2. A material according to claim 1, in which the LLDPE has a density less than 0.93g/cm2. 3. A material according to claim 1, in which the LLDPE has a T m value from 100 to 130°C. 4. A method according to claim 1, in which the LLDPE has a molecular weight distribution of from 2 to 6. 5. A material according to claim 1, in which the LLDPE has a melt flow index of from 0.7 to 10. 6. A material according to claim 1, in which the LLDPE is crosslinked. 7. A material according to claim 1, which is heat shrinkabie by virtue of the heat shrinkabie fibres comprising LLDPE. 8. A material according to claim 1, in which the fibres consist substantially only of LLDPE. 9. A material according to claim 1, in which the fibres form part of a fabric. 10. A material according to claim 9, in which the fabric additionally comprises non-heat shrinkabie fibres. 11. A material according to claim 1, which comprises the fibres and a matrix material that renders the material substantially impervious to liquids. 12. A method of environmentally protecting a substrate, which comprises positioning around the substrate a material according to any of claims 1 to 11, and causing heat shrinkage of the material 13. A method according to claim 12, in which the heat shrinkage is caused by heating the material with a hot air gun. 14. A material according to claim 12 or 13, in which the substrate comprises a telecommunications cable or a splice therein. * * * * * * *

Inventors: HUTT NORMAN, HUTT, NORMAN
Applicants: HUTT NORMAN, RAYCHEM LTD, HUTT, NORMAN, RAYCHEM LIMITED
Application year: 1994
Main classification: G02B6
IPC codes: C08L23, D01F6, G02B6